1. Field of the Invention
The present invention concerns an integrated circuit having an output buffer with improved protection against electrostatic discharge.
2. Description of the Prior Art
Numerous techniques have been developed to protect the input circuitry in integrated circuits from electrostatic discharge (ESD). In particular, the gates of insulated gate transistors are known to be susceptible to damage from an ESD event. In order to protect the inputs, various combinations of clamping diodes, transistors, and resistors are used between the input bondpad and the input transistors to limit the voltage that appears at the input gates. In the past, it has generally been assumed that output buffers, with their relatively low output impedance as compared to input gates, were much less susceptible that input gates to ESD damage. However, it has recently been found that the output buffers of MOS integrated circuits may also be quite susceptible to damage from ESD events. This appears to be especially true when the output buffers have a metal silicide contact layer on the gate and the source/drain diffused regions; see, for example, "ESD Phenomena and Protection Issues in CMOS Output Buffers" by C. Duvvury et al, IEEE International Reliability Physics Symposium at pages 174-180(1987). Although protective clamping diodes have also been used for the protection of output buffers, these have not provided as high an amount of protection as desired in many cases.
One problem with protecting output buffers is that the value of the resistors traditionally used to protect input circuitry is typically on the order of hundreds or even thousands of ohms. Such a value is much too high for use with output buffers, since the buffer must sometimes drive large capacitive loads (e.g., typically greater than 10 picofarads, and often greater than 100 picofarads). Therefore, an output resistor on the order of several hundred ohms (or more) would reduce the output switching speed of the buffer to an unacceptably slow value. In addition, the sinking and sourcing current capability of the buffers would be degraded with the use of a large value resistor. For these reasons, it has been considered desirable to use as good a conductor as possible between the output buffer and the bondpad, in order to allow the external load to be switched at high speed. This normally means that the metal (e.g., aluminum) layer of the integrated circuit is used to form the conductor between the output buffer and the bondpad. However, in some cases wherein only a single metal level is present on the integrated circuit, layout considerations have caused the underlying polysilicon or metal silicide layer to be used for this purpose. That is, when an aluminum conductor is required to overlie the output buffer, as for a power bus, a silicide layer is used to connect the output buffer to the bondpad. However, unless required for this or other purposes, the use of a polysilicon or silicide link between the output buffer and the bondpad has been avoided in the prior art, to preserve high performance.